In recent years, there have been proposed a wide variety of broadband circuit elements each of which is available for the realization of ultrahigh-speed/high-capacity wireless communication device. In particular, development of a broadband circuit element which covers the 59 to 66 GHz band is important. With this development, it is possible to realize an ultrahigh-speed wireless LAN, a home link, cable TV wireless transfer, an inter-vehicle communication system and other applications at a transmission rate exceeding, e.g., 400 Mbps.
As such a millimeter-wave or microwave transmission circuit, an NRD guide has been conventionally used (see JP-A-2000-341003). In this NRD guide, as shown in FIG. 17(a), a dielectric waveguide 101 formed of, e.g., Teflon® (registered trademark for polytetrafluoroethylene) having, e.g., a dielectric constant ∈r=2.04 is provided between a pair of parallel conductor plates 102a and 102b. A width of each of these conductor plates 102a and 102b, i.e., a height of the dielectric waveguide 101 is set to be less than a ½ wavelength of a frequency of an electromagnetic wave propagated through this dielectric waveguide 101, and a width of the dielectric waveguide 101 is set to be approximately a ½ wavelength. For example, if an operating frequency is 60 GHz, a height of the dielectric waveguide 101 is set to 2.25 mm and a width of the dielectric waveguide 101 is set to 2.5 mm. As a result, an electromagnetic wave having the operating frequency can be propagated through the dielectric waveguide 101, but the electromagnetic wave having the operating frequency cannot be propagated outside the dielectric waveguide 101 in a widthwise direction of the dielectric waveguide 101, and hence the electromagnetic wave having the operating frequency is trapped in and transmitted through the dielectric waveguide 101.
Although an electromagnetic field in a cross section is generated in an operating mode (an LSM mode) of the electromagnetic wave having the operating frequency transmitted through this dielectric waveguide 101 as shown in FIGS. 17(a) and 17(b), an LSE mode which is an unnecessary parasitic mode is produced due to bending or branching of the dielectric waveguide 101 as shown in FIG. 17(b).
In order to suppress this LSE mode, a mode suppressor 103 having a ¼ wavelength choke configuration is inserted into the dielectric waveguide 101 in the prior art as shown in FIG. 18.